(Assuming MM is behind pianorarescores and similar sites.)
I can't speak for Alistair, but my impression was that many years ago he had to take legal action to stop MM selling Sorabji scores. I've never seen anything other than negative impressions of his behaviour, and if someone is basically downloading bulk torrents of scores and videos, also acquiring the entire contents of collector websites, then selling them on for profit (plus many scores freely available through IMSLP, etc) I can't say it inspires confidence that he's going to be a model of business probity if one were to enter into a relationship of that type with him.
Hi Ronde_des_sylphes,
We really don't know very much about Marcel Mombeek and his history, his side of the story (or stories), his motivations, et c. It would be good to know more and to get his observations on all of this.
As far as I.M.S.L.P. goes, it is an archive of public domain music content and editions. People can do whatever they want with the material [within reason, of course! - if you "paper" your neighbor's house with Thalberg etudes, the police might have something to say about it!

]. If Marcel Mombeek has broken the law regarding some content, that is between him and the content owner (or owners).
And if such law breaking did happen, and if it was years ago, then how relevant and important would it be now in 2015?
If every person who has ever broken any law were to be barred from business relationships and employment, the unemployment rate would be almost 100%!
Mvh,
Michael